Natural and other gases, such as landfill gas, syngas, and geothermal gas, in the United States often contain hydrogen sulfide as a major contaminant. Hydrogen sulfide is a toxic, acid gas that is corrosive in the presence of water. A significant portion of the gas produced in the U.S. does not meet pipeline standards and requires treatment to reduce the concentration of hydrogen sulfide to 1/4 grain per 100 standard cubic feet or .ltoreq.4 ppm on a volume basis. Other sulfur compounds which may occur in these gases include carbonyl sulfide, carbon disulfide, and mercaptans, such as dimethyl mercaptan, methyl mercaptan, and ethyl mercaptan. While sulfur dioxide is occasionally present, sulfur dioxide is not one of the common contaminants in these gases.
Conventional commercial processes for removing gaseous sulfur species from a given gas stream are based upon using at least two groups of chemicals. One group is amine-based reagents, while the second group comprises liquid redox reagents. Both groups of chemicals need to be regenerated using additional process steps. Another limitation of these processes is that they are not cost effective at low throughput (amine, less than 100 million standard cubic feet per day, or &lt;100 MMSCFD, and liquid redox at &lt;5 MMSCFD) of the contaminated gases. Still other limitations are related to the operation of a gas-processing plant when chemical reagents, such as a liquid redox catalyst, are used.
Another group of chemical reagents, called Scavengers can be used for low-volume gas production facilities. The limitations, however, include (i) only one-time throughput of the reagent, (ii) high cost of the reagents, and (iii) production of hazardous wastes that require costly disposal.
Biological processes can overcome the limitations of processes based on chemical reagents. Commonly known biological processes are of two types. In the first type, a chemical reagent, such as LO-CAT catalyst, which principally comprises chelated iron sulfate, oxidizes hydrogen sulfide to elemental sulfur. Subsequently, the spent catalyst is regenerated using microorganisms, such as Thiobacillus ferrooxidans, rather than air oxidation. In this way, the process is much safer and the power requirements are reduced, resulting in a more economical overall process.
These microorganisms carry out the regeneration of the LO-CAT catalyst according to the following reaction: ##STR1##
A recent patent to Rai, U.S. Pat. No. 5,508,014, teaches that the microorganism Thiobacillus ferrooxidans regenerates the LO-CAT catalysts at much higher rates than can be achieved through air oxidation.
The second type of biological process is the direct treat process, in which bacteria oxidize the sulfur species by using the sulfur species as an energy source. This reaction is carried out in the presence of the following components:
a terminal electron acceptor such as NO.sub.3.sup.-1 ; PA1 a source of carbon, such as carbon dioxide, present in the gas stream, or HCO.sub.3.sup.-1, present in the nutrient solution (culture medium) for the growth of bacteria; and PA1 NH.sub.4.sup.+1 as a source of reduced nitrogen.
The predominant biochemical reaction underlying the direct treat biological process is as follows: EQU 0.422H.sub.2 S+O.422HS.sup.-1 +NO.sub.3.sup.-1 +0.342CO.sub.2 +0.0685HCO.sub.3.sup.-1 +0.0865NH.sub.4.sup.+1.fwdarw. EQU 0.844SO.sub.4.sup.-2 +0.5N.sub.2 +0.288H.sup.+1 +0.4025H.sub.2 O+0.0865C.sub.8 H.sub.7 O.sub.2 N (biomass) (iii)
There are a number of chemolithotrophic bacteria that oxidize elemental sulfur and use reduced or partially reduced sulfur compounds as an energy source, carbon dioxide or bicarbonate as a carbon source, and ammonium ion as a source of reduced nitrogen. For example, the anaerobic photosynthetic bacterium Chlorobium thiosulfatophilum is used to convert sulfides to sulfate (Cork et al, 1982). Since the process must be conducted under photosynthetic conditions, the capital and operating costs for this process are economically unattractive.
Another commonly known chemolithotrophic microorganism is the aerobic bacterium Thiobacillus denitrificans. One process for desulfurizing sour natural gas using this bacterium is disclosed in Sublette, U.S. Pat. No. 4,760,027. This patent describes a process wherein bacteria of the Thiobacillus genus convert sulfides to sulfates under aerobic conditions and at a controlled temperature of about 30.degree. C.
A review of the literature, however, reveals that, in contrast to a homogeneous culture consisting of only one bacterial species (e.g., Thiobacillus ferrooxidans or T. denitrificans), a mixed microbial culture (consortium) composed of compatible bacteria or microorganisms of different biochemical and morpho-physiotypes working in synergy bring about 75% higher sulfide oxidation than a chemical agent. The important condition, nevertheless, is that in this system the sulfide concentration should be .ltoreq.3-8 mM and reaction conditions should be favorable for the microbial metabolism.
The above-described direct treat microbial process for direct removal of hydrogen sulfide from methane or fuel gas streams containing other hydrocarbons, however, poses a potential danger of explosion when, for example, methane and air are mixed. ARCTECH has developed an anaerobic, non-photosynthetic biological process, described in application Ser. No. 08/651,793, in which a microbial consortium, SSII (also known as ATCC 202177) from ARCTECH's Microbial Culture Collection (AMCC) reduces the hydrogen sulfide concentration from up to 10,000 ppm to less than 4 ppm. The SSII microbial consortium was deposited in the American Type Culture Collection (ATCC) on Oct. 6, 1998 with the ATCC Patent Depository. The ATCC is located at 10801 University Blvd., Manassas, Va. 20110. The deposit number of the SSII microbial consortium is ATCC 202177. ATCC 202177 oxidizes hydrogen sulfide to elemental sulfur, which is neither corrosive nor toxic.
There is a critical need for a cost-effective system for removing gaseous sulfur species in the presence of other gases, even in small-volume operations. It is critical, then, to minimize chemicals and nutrients, produce useful by-products which are not hazardous, and eliminate a separate regeneration step of the catalyst.
The present invention is related to a method for preparing and enhancing the ability of ATCC 202177, also known as SSII or DSC2, used in the parent application to remove hydrogen sulfur at high rates. Specifically, the present invention is related to a preparation of ATCC 202177 which is designed to remove carbon dioxide and gaseous sulfur species, such as hydrogen sulfide, carbon disulfide, and mercaptans, in the presence of methane, hydrogen, carbon monoxide, and other gases.